Antisera were prepared in rabbits against the idiotypic (Id) determinants of antiallergen antibodies. These antibodies were isolated from the plasma of 10 unrelated patients by immunoabsorption. Three major allergens isolated from the house dust mite, Dermatophagoides pteronyssinus (DPT), were used. The anti-Id antisera were rendered specific by successive absorption on insolubilized allergens, IgG and IgM from a pool of human sera and Ig from the donor after removal of anti-DPT antibodies by absorption. Anti-Id antibodies raised essentially against IgG antibodies were found to inhibit the agglutinating activity of both IgG and IgM anti-DPT antibodies toward allergen-coated latex. These anti-Id antibodies were also able to inhibit partly the binding of radiolabeled allergen to IgE anti-DPT antibodies. Matching the anti-Id antisera with the various individual anti-DPT antibodies revealed an Id cross-reactivity of about 71% for IgG, 61% for IgM and 62% for IgE. A second and predominant type of anti-Id antibodies recognized bystander idiotopes as the allergen did not inhibit Id-anti-Id reaction. These idiotopes were apparently more specific to individuals (private) as the anti-DPT antibodies of a given patient did not agglutinate latex particles coated with anti-Id antibodies prepared against Id of another patient.